Surface treatment compositions and coated articles prepared therefrom

ABSTRACT

Surface treatment compositions are provided comprising:
         (a) a fluorine-containing polymer;   (b) a solvent containing at least one C—F bond; and   (c) a rheology modifying component. Also provided are coated articles, comprising:   (i) a substrate; and   (ii) a self-assembled coating layer applied to at least one surface of the substrate. The self-assembled coating layer is formed from the surface treatment composition above.

FIELD OF THE INVENTION

The present invention relates to surface treatment compositions thatdemonstrate thixotropy and to coated articles prepared therefrom.

BACKGROUND OF THE INVENTION

The use of coatings to protect electronic circuitry from externalenvironments is widely known and many different treatments exist toperform that function. However, there are currently no liquid,solution-based coatings that can provide a uniform coating thicknessover the complex geometries of printed circuit boards. Chemical vapordeposition (e. g., vacuum-applied technologies) such as the applicationof parylene can make a true conformal coating. Unfortunately,vacuum-deposited coatings such as parylene-based treatments usuallyrequire labor-intensive masking of “keep-out” areas.

Conventional conformal coating solutions used on circuit boards willflow and level to some degree without being under external shear forcesbesides gravity. This causes the corners, edges, sidewalls to have amuch thinner coating than the flat/recessed areas of the coatedsubstrate, and when the coated circuitry is exposed to water while poweris on, failure occurs at these areas. Therefore, in order for a printedcircuit board to survive immersion in water while being powered up, acoating must be applied that will have uniform coverage regardless ofsurface topology.

It would be desirable to provide surface treatment compositions thatdeliver the benefits of conformal coatings yet eliminate the need forcostly capital investment and avoid the bottlenecking caused byvacuum-based batch manufacturing processes or masking operations.

SUMMARY OF THE INVENTION

Surface treatment compositions are provided comprising:

(a) a fluorine-containing polymer;(b) a solvent containing at least one C—F bond; and(c) a rheology modifying component. Also provided are coated articles,comprising:(i) a substrate; and(ii) a self-assembled coating layer applied to at least one surface ofthe substrate; wherein the coating layer is formed from the surfacetreatment composition above.

DETAILED DESCRIPTION OF THE INVENTION

Other than in any operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions and soforth used in the specification and claims are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

As used in this specification and the appended claims, the articles “a,”“an,” and “the” include plural referents unless expressly andunequivocally limited to one referent.

The various aspects and examples of the present invention as presentedherein are each understood to be non-limiting with respect to the scopeof the invention.

The present invention provides surface treatment compositions that formself-assembled structures upon application to a substrate, and result incoatings of uniform thickness regardless of the surface topology of thearticle being coated. It has been discovered that particularcombinations of polymer, solvent and rheological modifier can yield acoating composition that provides the correct balance of the delicateintramolecular interactions between the various components in order togenerate self-assembled structures. These self-assembled structuresprovide thixotropy to the composition and thus allow it to demonstratedramatic changes in viscosity as a response to external stimuli,particularly the application of shear forces. Most importantly, thesecompositions will rapidly increase in viscosity when shear forces areremoved, which allows for the coating composition to adhere to complexshapes without running (sag) or flowing. The resultant dried film willhave a much greater uniformity in thickness compared to a coatingcomposition that does not form such structures.

The surface treatment compositions of the present invention comprise (a)a fluorine-containing polymer. The polymers may be any polymers thatcontain fluorocarbon (i. e., C—F) units, such as —C(F)₂—, —C(F)(H)—,and/or terminal units such as —C(F)_(x)(H)_(y), wherein x is greaterthan or equal to 1 and x+y=3. Note that the phrase “and/or” when used ina list is meant to encompass alternative embodiments including eachindividual component in the list as well as any combination ofcomponents. For example, the list “A, B, and/or C” is meant to encompassseven separate embodiments that include A, or B, or C, or A +B, or A +C,or B +C, or A +B +C. The fluorine-containing polymer (a) is most often a(meth)acrylic polymer. By “(meth)acrylic” is meant polymers preparedfrom monomers having acrylic functional groups, polymers prepared frommonomers having methacrylic functional groups, and/or polymers preparedfrom both types of monomers. The polymers are typically prepared fromacrylic and methacrylic monomers such as acrylic acid, methacrylic acid,and esters thereof. Useful alkyl esters of acrylic acid or methacrylicacid include aliphatic alkyl esters containing from 1 to 30, and often 4to 18 carbon atoms in the alkyl group. Non-limiting examples includemethyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylacrylate, butyl acrylate, and 2-ethyl hexyl acrylate. Suitable othercopolymerizable ethylenically unsaturated monomers include vinylaromatic compounds such as styrene and vinyl toluene; nitriles such asacrylonitrile and methacrylonitrile; vinyl and vinylidene halides suchas vinyl chloride and vinylidene fluoride, vinyl esters such as vinylacetate or ethers such as vinyl trifluoromethyl ether. Fluoro-functionalmonomers such as monofluoroethylene, difluoroethylene,trifluoroethylene, and tetrafluoroethylene are suitable for impartingfluoro functionality to the polymer.

(Meth)acrylic polymers can be prepared via known organic solutionpolymerization techniques. Generally any method of producing suchpolymers that is known to those skilled in the art utilizing artrecognized amounts of monomers can be used.

The fluorine-containing polymer (a) typically demonstrates a weightaverage molecular weight (Mw) of 5,000 to 500,000; usually 25,000 to250,000 Daltons as measured by gel permeation chromatography using apolystyrene standard. Particularly suitable methacrylic polymers includeI500101, commercially available from Aculon, Inc.

The fluorine-containing polymer (a) is usually present in the surfacetreatment composition in an amount of 5 to 25 percent by weight, moreoften 12.5 to 17.5 percent by weight, based on the total weight of thesurface treatment composition.

The surface treatment compositions of the present invention furthercomprise (b) a solvent containing at least one C—F bond. Examplesinclude hydrofluoroether (HFE) solvents. Such solvents were developedoriginally as replacements for CFCs, HFCs, HCFCs, and PFCs. An advantageof using an HFE solvent is its short atmospheric lifetime and zero ozonedepletion potential compared to alternative chemicals.

Examples of particular hydrofluoroether solvents include1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane and/or1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane, commercially available from3M Corporation as NOVEC 7200. Other exemplary solvents include3-ethoxyperfluoro(2-methylhexane) (HFE 7500, also available from 3MCorporation); 1H,1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether(HFE 6512, available from Fuxin Hengtong); and/or1,1,1,2,3,4,4,5,5,5-Decafluoropentane (Vertrel XF, available from E. I.DuPont de Nemours).

The solvent (b) is usually present in the surface treatment compositionin an amount of 75 to 90 percent by weight, more often 75 to 89 percentby weight, based on the total weight of the surface treatmentcomposition.

The surface treatment compositions of the present invention furthercomprise (c) a rheology modifying component. Fumed silica that issurface modified with hydrophobic functional groups is a particularlysuitable rheology modifying component. The fumed silica contributes tothe thixotropic nature of the surface treatment composition of thepresent invention. By “hydrophobic” is meant that the functional groupshave non-polar properties and have a tendency to interact with, bemiscible with, or be dissolved by non- polar solvents such as alkanesand oils.

Fumed silica is formed from microscopic droplets of amorphous silicafused into branched, chainlike, three-dimensional secondary particleswhich then agglomerate into tertiary particles. It is manufactured byflame pyrolysis of silicon tetrachloride or from quartz sand vaporizedin a 3000° C. electric arc. The resulting powder has an extremely lowbulk density (often 160-190 kg/m3) and high surface area (usually 50-600m2/g). Hydrophobic functional groups with which the silica particlesurfaces may be modified include hexamethyldisiloxane,dichlorodimethylsilane, trimethylchlorosilane,octadecyldimethylchlorosilane, chlorinated poly(dimethylsiloxane), andthe like. Particularly suitable fumed silica is commercially availablefrom Cabot Corporation and Evonik Industries.

The rheology modifying component (c) is usually present in the surfacetreatment composition in an amount of 1 to 5 percent by weight, moreoften 1.5 to 2.5 percent by weight, based on the total weight of thesurface treatment composition.

The surface treatment compositions of the present invention typicallydemonstrate a thixotropic index greater than 2. Thixotropic index isdefined as the ratio of the viscosity under slow speed shear to theviscosity under high speed shear of a non-Newtonian fluid. It is used asa tool to define how well the fluid will hang or resist sagging undergravity. The thixotropic index is obtained by measuring the viscosity ofa fluid at ambient temperature. Ambient temperature typically rangesfrom 60 to 90° F. (15.6 to 32.2° C.), such as a typical roomtemperature, 72° F. (22.2° C.). A viscometer is used to measure theviscosity at two speeds (one is a multiple or a factor of 10 of theother). The index is calculated by dividing the viscosity at the lowerspeed shear by the viscosity at the higher speed shear.

In a particular example of the present invention, the surface treatmentcomposition comprises (a) a (meth)acrylic fluorine-containing polymer;(b) a hydrofluoroether solvent; and (c) fumed silica that is surfacemodified with hydrophobic functional groups.

The surface treatment compositions of the present invention are suitablefor use in preparing coated articles such as coated circuit boards. Thecoated articles of the present invention comprise a substrate (i).Substrates suitable for use in the preparation of the coated articles ofthe present invention can include a metal such as copper or steel, orany substrate commonly used in the preparation of circuit assemblies,such as polyepoxides, including fiberglass reinforced polyepoxides,polyimides, phenolics, and fluorocarbons. The polymeric substrates maycomprise an electrically conductive circuit pattern imprinted in (suchas in the case of a blind via) or on the substrate surface. Often thesubstrate is an electronic component of a circuit assembly, such as aprinted circuit board.

The substrate may take any shape as desired for the intendedapplication, such as flat, curved, bowl-shaped, tubular, or freeform.For example, the substrate may be in the form of a flat plate having twoopposing surfaces, such as would be suitable for use in an electroniccircuit assembly as a circuit board.

Prior to application of any coatings, the substrate may be cleaned suchas by argon plasma treatment or with a solvent such as lonox 13416 orCybersolv 141-R, both available from Kyzen.

The coated articles of the present invention further comprise a self-assembled coating layer (ii) applied to at least one surface of thesubstrate. The coating layer comprises any of the surface treatmentcompositions described above.

The coating layer may be applied to the substrate by one or more of anumber of methods such as spraying, dipping (immersion), spin coating,or flow coating onto a surface thereof.

The self-assembled coating layer typically demonstrates a dry filmthickness (DFT) of 5 to 100 microns. The coating layer demonstrates avery uniform thickness over the surface of the substrate and serves as aconformal coating. The dry film thickness of the coating layer over thesurface of the substrate typically varies by no more than 50 percent.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the scope of the inventionas defined in the appended claims.

What is claimed is:
 1. A surface treatment composition comprising: (a) afluorine-containing polymer; (b) a solvent containing at least one C—Fbond; and (c) a rheology modifying component.
 2. The surface treatmentcomposition of claim 1 wherein the fluorine-containing polymer (a) ispresent in an amount of 10 to 20 percent by weight, based on the totalweight of the surface treatment composition.
 3. The surface treatmentcomposition of claim 1 wherein the fluorine-containing polymer (a)comprises a (meth)acrylic polymer.
 4. The surface treatment compositionof claim 1, wherein the fluorine-containing polymer (a) has a weightaverage molecular weight of 25,000 to 500,000.
 5. The surface treatmentcomposition of claim 1 wherein the solvent (b) is present in an amountof 75 to 89 percent by weight, based on the total weight of the surfacetreatment composition.
 6. The surface treatment composition of claim 1,wherein the solvent (b) comprises 3-ethoxyperfluoro(2-methylhexane),1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane,1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane,1,1,1,2,3,4,4,5,5,5-Decafluoropentane and/or 1H ,1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.
 7. The surfacetreatment composition of claim 1 wherein the rheology modifyingcomponent (c) is present in an amount of 1 to 5 percent by weight, basedon the total weight of the surface treatment composition.
 8. The surfacetreatment composition of claim 1 wherein the rheology modifyingcomponent (c) comprises fumed silica that is surface modified withhydrophobic functional groups.
 9. The surface treatment composition ofclaim 1 wherein said surface treatment composition demonstrates athixotropic index greater than
 2. 10. A coated article comprising: (i) asubstrate; and (ii) a self-assembled coating layer applied to at leastone surface of the substrate; wherein the coating layer is formed from asurface treatment composition comprising: (a) a fluorine-containingpolymer; (b) a solvent containing at least one C—F bond; and (c) arheology modifying component.
 11. The coated article of claim 10 whereinthe substrate (i) comprises a printed circuit board.
 12. The coatedarticle of claim 10, wherein the coating layer (ii) demonstrates a dryfilm thickness of 5 to 100 microns.
 13. The coated article of claim 12,wherein the dry film thickness of the coating layer over the surface ofthe substrate varies by no more than 50 percent.
 14. The coated articleof claim 10 wherein the fluorine-containing polymer (a) is present inthe surface treatment composition in an amount of 10 to 20 percent byweight, based on the total weight of the surface treatment composition.15. The coated article of claim 10 wherein the fluorine-containingpolymer (a) comprises a (meth)acrylic polymer.
 16. The coated article ofclaim 10 wherein the fluorine-containing polymer (a) has a weightaverage molecular weight of 25,000 to 500,000.
 17. The coated article ofclaim 10 wherein the solvent (b) is present in the surface treatmentcomposition in an amount of 75 to 89 percent by weight, based on thetotal weight of the surface treatment composition.
 18. The coatedarticle of claim 10 wherein the solvent (b) comprises3-ethoxyperfluoro(2-methylhexane),1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane,1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane, 1,1,1,2 ,3 ,4 ,4 ,5 ,5,5-Decafluoropentane and/or 1H,1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.
 19. The coatedarticle of claim 10 wherein the rheology modifying component (c) ispresent in the surface treatment composition in an amount of 1 to 5percent by weight, based on the total weight of the surface treatmentcomposition.
 20. The coated article of claim 10 wherein the rheologymodifying component (c) comprises fumed silica that is surface modifiedwith hydrophobic functional groups.